The invention relates to a process for producing ceramic-metal composites, ceramic-metal composites and their use.
Conventional brake discs are generally made of grey cast iron. A tried and tested technology exists for this, resulting in a low cost level in mass production. However, problems increasingly occur in modern vehicles, since ever greater masses have to be braked at rising maximum speeds. This situation considerably increases the structural complication of the brake systems (e.g. internal ventilation), since heating of the brake disc to above a critical temperature has to be prevented. An increase in the size of braking systems also increases their mass, which has an adverse effect on handling.
A further group of materials already in use for brake applications are the MMCs (metal matrix composites). These are generally particle-reinforced aluminium, with the reinforcing phase usually consisting of a ceramic such as SiC or Al.sub.2 O.sub.3. For this class of materials, a low density is a key property, which enables components having a low weight to be realized. However, this material reaches its thermal limits at about 450.degree. C., since the aluminium base material very quickly softens and loses its mechanical strength.
The lowest density of all potential brake materials is displayed by the carbon fibre-reinforced carbons (C/C). These materials have already been used successfully for some time in racing. Their disadvantage is primarily their high wear which would make them uneconomical for production vehicles. In addition, C/Cs have a low thermal conductivity and heat capacity which leads to them heating up to a high temperature during braking.
Infiltration of C/C materials with silicon gives a ceramic SiC material which is reinforced by carbon fibres. This is likewise an interesting brake material, but in its present form it has a low coefficient of friction and, like all fibre-reinforced materials, is still very expensive.
Processes for producing ceramic-metal composites by infiltration of porous ceramic bodies have already been described in a number of patent documents.
U.S. Pat. No. 5,535,857 claims the production of a ceramic-metal brake disc by infiltration of a porous SiC precursor body. For this ceramic body, the SiC powder is pressed into the required shape and presintered, so that open pore channels remain. The porous disc is then infiltrated with an aluminium alloy, forming a metal-reinforced ceramic matrix. The metal undergoes no reaction with the matrix during infiltration, so that the heat resistance of the material depends on the reinforcing matrix. In the case of infiltration with aluminium, this means that the use limit of the material is 400.degree. C.
In a further process, the infiltration of a ceramic precursor body with aluminium has likewise been described (U.S. Pat. No. 4,988,645). Here, the ceramic body is produced by means of an SHS reaction (SHS=self-propagating high-temperature synthesis, in which a reactive mixture is ignited and undergoes a self-propagating reaction to give the desired ceramic matrix as reaction product).
U.S. Pat. No. 4,033,400 claims the infiltration of a porous ceramic body with a liquid metal, where the matrix comprises Si.sub.3 N.sub.4 and the metal is an aluminium alloy. Here too, it is clearly stated as important that no reaction should take place between the matrix and the metal.
Lanxide Technology likewise claims a series of materials which have been produced by metal infiltration (e.g. EP-B-0 368 785, EP-B-0 368 784). These patents essentially claim new process steps such as the targeted oxidation of the ceramic precursor body.
In all patents hitherto, no reaction infiltration took place. A single exception is the patent U.S. Pat. No. 4,585,618 which discloses the only process in which the infiltrated metal (aluminium) undergoes a reaction with the matrix. The aim of this invention is to produce a reinforced TiB.sub.2 /Al.sub.2 O.sub.3 ceramic for electrolysis cells. For this purpose, a TiO.sub.2 /B.sub.2 O.sub.3 /Al mixture is infiltrated with aluminium. The infiltration time is 100 hours! The reaction product comprises TiB.sub.2 /Al.sub.2 O.sub.3 /Al, with Al.sub.3 Ti also being detected on the surface, although this is not desired.
At this point, reference is made to the patent application DE-P 197 06 925 Al entitled "Process for producing ceramic-metal composites, ceramic-metal composites and their use" submitted by the applicant and having the same priority as the present patent application.
The process described here differs from the existing processes primarily in that the infiltration with aluminium results in a reaction which produces a high-temperature-resistant aluminium alloy. In addition to the reaction infiltration, the ceramic precursor body is also produced by a reaction synthesis, by which means the future material composition can be controlled in terms of its functional constituents.
The material of the present invention has a density of 3.4 g/cm.sup.3, which is slightly higher than the density of the MMCs, but only 42% of the density of cast iron. As a result of the high-temperature-resistant phases of TiAl, its use range should extend up to 800.degree. C.; it should thus significantly exceed the values for grey cast iron.
A further important advantage of this material is the low costs both of the raw materials and of the process technology. The material and the process have the potential to give a price per item in the vicinity of that of cast iron discs in mass production.
The process of the invention is described in detail below.